The incident happened about 3 a.m. Saturday at the St. George home of Scot and Wanda Denhalter, The Salt Lake Tribune reported.
The couple moved into the rental home last month despite Scot's fears a big rock from the boulder-laden ridge looming above the back yard could one day loosen and come tumbling down upon them.
"She said, 'No it won't,'" said Scot, 62.
Scot estimated the boulder that hit his house was about 12 feet long, 9 feet high and 9 feet thick. He said his wife narrowly escaped being crushed by it.
"I guess she heard the rumbling as it came down the hill and woke up," said Scot, who was staying at his son's nearby home overnight. "She rolled away from the oncoming noise to my side of the bed. If she had decided to swing her legs over the side of the bed and get up to investigate, it would have killed her."
Wanda, 63, was struck by the rock, though, and suffered a broken jaw and sternum. The skin on one of her legs was ripped open.
"She's very lucky," her husband said.